Interest in image sensors capable of three-dimensional (3D) imaging is increasing as the popularity of 3D applications continues to grow in applications such as imaging, movies, games, computers, user interfaces, and the like. A typical passive way to create 3D images is to use multiple cameras to capture stereo or multiple images. Using the stereo images, objects in the images can be triangulated to create the 3D image. One disadvantage with this triangulation technique is that it is difficult to create 3D images using small devices because there must be a minimum separation distance between each camera (ideally, approximating the human eye separation) in order to create the three dimensional images. In addition, this technique is complex and therefore requires significant computer processing power in order to create the 3D images in real time.
For applications that require the acquisition of 3D images in real time, active depth imaging systems based on the optical time of flight measurement are sometimes utilized. These time of flight systems typically employ a light source that directs light at an object, a sensor that detects the light that is reflected from the object, and a processing unit that calculates the distance to the object based on the round trip time that it takes for light to travel to and from an object. In typical time of flight sensors, photodiodes are often used because of the high transfer efficiency from the photo detection regions to the sensing nodes. Some known time of flight sensors need larger pixel sizes to collect an acceptable signal level from the light (which is often low intensity and short duration light) reflected off of the object. Some known time of flight sensors accumulate and store charge through multiple accumulations of the light from the light source to attain higher signal levels. However, leakage current may drain the stored charge during the multiple accumulations of the light, leaving poor signal to noise ratios.